Joergen Christensen-Dalsgaard

Context. Asteroseismology provides precise stellar parameters, which are particularly useful for benchmark stars with accurate independent measurements. However, most benchmark solar-like oscillators are red giants. Aims. We present a detailed analysis of the asteroseismic main-sequence benchmark star χ Dra A in the Transiting Exoplanet Survey Satellite (TESS) northern continuous viewing zone. We aim to derive its stellar mass and radius from asteroseismic modelling of individual mode frequencies and test these derived values against an independent dynamical mass and interferometric radius measurements. Methods. We determined the dynamical mass of χ Dra using 618 radial velocity measurements obtained with the SONG telescope at Tenerife. Our results were derived via the spectral disentanglement method, and 53 relative astrometric measurements. From the disentangled spectra, we also determined the spectroscopic parameters of χ Dra A. With the PAVO beam combiner at CHARA, we obtained the interferometric radius of χ Dra A. We determined asteroseismic parameters from 16 sectors of 20 sec cadence TESS photometry. Results. We determine T _eff = 6277 ± 30 K, [Fe/H] = −0.51 ± 0.03 dex, and [ α /Fe] = 0.08 ± 0.03 dex from the spectroscopic analysis. For the dynamical fit, we obtain a mass of M _A = 1.028 ± 0.004 M _⊙ for χ Dra A and M _B = 0.735 ± 0.003 M _⊙ for χ Dra B. Combining the derived interferometric angular diameter with dynamical parallax yields an interferometric radius of R _A = 1.159 ^+0.029 _−0.028 R _⊙ , the first interferometric radius of χ Dra A. In the TESS power spectrum, we identify 38 individual oscillating modes. Using these modes, we modelled the star with nine independent pipelines to test the resulting model mass and radius against our independently calculated mass and radius. All models yield masses slightly lower than the dynamical mass. Using asteroseismic scaling relations, we find that scaling masses from corrected scaling relations best reproduce the dynamical mass. Conclusions. The combination of spectroscopy, interferometry, and asteroseismology yields precise results for the main component of the χ Dra system, making it one of the best-characterised main-sequence solar-like oscillators.

Context. The Stellar Observations Network Group (SONG) network has operated for more than a decade, providing long-baseline high-cadence spectroscopic observations of bright stars and the Sun. The observations from 2014 through 2025 constitute a substantial archive of high-resolution spectra and precise radial velocities suitable for a broad range of time-domain stellar astrophysics. Aims. We present an overview of the current status, instrumentation, and scientific capabilities of the SONG network and describe the scope and accessibility of the SONG data archive (SODA). We further illustrate the breadth of science enabled by SONG observations, including asteroseismology, stellar variability studies, binary-star characterisation, and exoplanet research. Methods. We summarise the operational status and observing strategies of the SONG facilities, describe the available data products and archive infrastructure, and outline procedures for accessing archival observations and proposing new observations within the SONG community framework. Results. The SODA archive currently contains more than 580, 000 spectra of 3091 stars obtained with SONG using either iodine-cell or thorium–argon wavelength calibration. The archive spans more than a decade of observations and includes extensive high-cadence time-series data for bright targets spanning a wide range of stellar types and variability classes. Access to the archive is available to members of the SONG community, which remains open to new participants who agree to follow the community policies. Conclusions. The SONG archive has developed into a major long-baseline resource for stellar spectroscopy and radial velocity time-series analysis. Continued expansion of the archive, together with coordinated observations obtained contemporaneously with TESS and future PLATO observations, is expected to enable new studies of stellar oscillations, variability, and exoplanet host stars through combined radial velocity and photometric analyses.

Aims. We present time-series radial velocities of the G8 subgiant star β Aql obtained in 2022 and 2023 using SONG-Tenerife and, for the first time, SONG-Australia. We also analyse a sector of TESS photometry that overlapped with the 2022 SONG data. Methods. We processed the time series to assign weights and to remove bad data points. The resulting power spectrum clearly shows solar-like oscillations centred at 430 μHz. The TESS light curve shows the oscillations at lower signal-to-noise, reflecting the fact that photometric measurements are much more affected by the granulation background than are radial velocities. Results. The simultaneous observations in velocity and photometry represent the best such measurements for any star apart from the Sun. They allowed us to measure the ratio between the bolometric photometric amplitude and the velocity amplitude to be 26.6 ± 3.1 ppm/ms⁻¹. We measured this ratio for the Sun from published SOHO data to be 19.5 ± 0.7 ppm/ms⁻¹ and, after accounting for the difference in effective temperatures of β Aql and the Sun, these values align with expectations. In both the Sun and β Aql, the photometry-to-velocity ratio appears to be a function of frequency. We also measured the phase shift of the oscillations in β Aql between SONG and TESS to be −113° ±7°, which agrees with the value for the Sun and also with a 3D simulation of a star with similar properties to β Aql. Importantly for exoplanet searches, we argue that simultaneous photometry can be used to predict the contribution of oscillations to radial velocities. We measured frequencies for 22 oscillation modes in β Aql and carried out asteroseismic modelling, yielding an excellent fit to the frequencies. We derived accurate values for the mass and age, and were able to place quite strong constraints on the mixing-length parameter. Finally, we show that the oscillation properties of β Aql are very similar to stars in the open cluster M67.

Stellar oscillation codes are software instruments that evaluate the normal-mode frequencies of an input stellar model. While inter-code comparisons are often used to confirm the correctness of calculations, they are not suitable for characterizing the numerical error of an individual code. To address this issue, we introduce a set of tools—“error measures”—that facilitate this characterization. We explore the behavior of these error measures as calculation parameters, such as the number of radial grid points used to discretize the oscillation equations, are varied, and we summarize this behavior via an idealized error model. While our analysis focuses on the GYRE code, it remains broadly applicable to other oscillation codes.

High-resolution helioseismology observations with the Helioseismic and Magnetic Imager (HMI) onboard the Solar Dynamics Observatory (SDO) provide a unique three-dimensional view of the solar interior structure and dynamics, revealing a tremendous complexity of the physical processes inside the Sun. We present an overview of the results of the HMI helioseismology program and discuss their implications for modern theoretical models and simulations of the solar interior.

Context. Metal-poor stars play a crucial role in understanding the nature and evolution of the first stellar generation in the Galaxy. Previously, asteroseismic characterisation of red-giant stars has relied on constraints from the global asteroseismic parameters and not the full spectrum of individual oscillation modes. Using the latter, we present for the first time the characterisation of two evolved very metal-poor stars including the detail-rich mixed-mode patterns. Aims. We will demonstrate that incorporating individual frequencies into grid-based modelling of red-giant stars enhances its precision, enabling detailed studies of these ancient stars and allowing us to infer the stellar properties of two very metal-poor [Fe/H] ∼ −2.5 dex Kepler stars: KIC 4671239 and KIC 7693833. Methods. Recent developments in both observational and theoretical asteroseismology have allowed for detailed studies of the complex oscillation pattern of evolved giants. In this work, we employ Kepler timeseries and surface properties from high-resolution spectroscopic data within a grid-based modelling approach to asteroseismically characterise KIC 4671239 and KIC 7693833 using the BAyesian STellar Algorithm, BASTA. Results. Both stars show agreement between constraints from seismic and classical observables, an overlap unrecoverable when purely considering the global asteroseismic parameters. KIC 4671239 and KIC 7693833 were determined to have masses of 0.78_−0.03^+0.04 and 0.83_−0.01^+0.03 M_⊙ with ages of 12.1_−1.5^+1.6 and 10.3_−1.4^+0.6 Gyr, respectively. Particularly, for KIC 4671239 the rich spectrum of model frequencies closely matches the observed. Conclusions. A discrepancy between the observed and modelled ν_max of ∼10% was found, indicating a metallicity dependence of the ν_max scaling relation. For metal-poor populations, this results in overestimations of the stellar masses and wrongful age inferences. Utilising the full spectra of individual oscillation modes lets us circumvent the dependence on the asteroseismic scaling relations through direct constraints on the stars themselves. This allows us to push the boundaries of state-of-the-art detailed modelling of evolved stars at metallicities far different from solar.

Context. In order to improve the asteroseismic modelling efforts for red-giant stars, the numerical computation of theoretical individual oscillation modes for evolved red-giant models has to be made feasible. Aims. We aim to derive a method for circumventing the computational cost of computing oscillation spectra for models of red-giant stars with an average large frequency separation Δν < 15 μHz, thereby allowing asteroseismic investigations of giants utilising individual frequencies. Methods. The proposed truncated scanning method serves as a novel method detailing how the observable individual l = 0, 1, 2 frequencies of red giants may be computed on realistic timescales through so-called model truncation. By removing the innermost region of the stellar models, the g-mode influence on the oscillation spectra may be avoided. This then allows us to estimate the observable regions from the resulting pure p-mode oscillations. The appropriate observable frequency regions are subsequently scanned for the complete and non-truncated stellar model. The observable regions are determined by considering the limitations on observability from the internal mode coupling and damping, yielding consistent frequency spectra obtained at a much reduced computational cost. Results. The truncated scanning method proves that the individual frequencies of red-giant models can be obtained for a wide range of applications and research. The computational efficiency is improved by a factor of 10 or better. This means that the inclusion of l = 1, 2 individual frequencies is now a possibility in future asteroseismic modelling efforts of red-giant stars. Further potential avenues for improvements of this method are outlined for future pursuits.